The present invention relates to a semiconductor device that functions as a heterojunction bipolar transistor, and in particular to measures for suppressing variations in properties such as the current amplification factor.
In recent years, progress in miniaturization and self-aligning technology has led to smaller and faster bipolar transistors formed using a silicon substrate. Ordinary bipolar transistors are so-called homojunction bipolar transistors, which use a silicon substrate and a monocrystalline silicon layer epitaxially grown on the silicon substrate.
On the other hand, heterojunction bipolar transistors (hereinafter, referred to as xe2x80x9cHBTxe2x80x9d) are being actively researched and developed for the purpose of further increasing operating speeds. In particular recently, there has been a strong push for the development of HBTs in which a SiGe layer, which is a mixed crystal of silicon and germanium, is epitaxially grown on a silicon substrate, and the SiGe layer is taken as a base layer (hereinafter referred to as xe2x80x9cSiGe-HBTxe2x80x9d).
FIG. 8 is a cross-sectional view of a conventional SiGe-HBT. As shown in the drawing, conventional SiGe-HBTs are formed using a Si substrate 101 and a Si epitaxial layer 102 epitaxially grown on the Si substrate 101. They also include an N+ buried layer 110, which is provided spanning a portion of the Si substrate 101 and a portion of the Si epitaxial layer 102, and an N+ collector lead layer 111, which is provided by introducing a relatively high concentration of N-type impurities into a portion of the Si epitaxial layer 102. The portion of the Si epitaxial layer 102 other than the N+ collector lead layer 111 is an Nxe2x88x92 collector diffusion layer 112, which includes a low concentration of N-type impurities. Furthermore, LOCOS isolations 116 separating the Si epitaxial layer 102 into the bipolar transistor formation regions, and deep trench isolations 117, which extend downward from the LOCOS isolations 116 and into the Si substrate 101, are also provided. However, in the bipolar transistor formation region there is no deep trench isolation 117 provided below the LOCOS isolation 116 separating the N+ collector lead layer 111 and the Nxe2x88x92 collector diffusion layer 112.
Furthermore, a SiGe film 108, which is made of a SiGe mixed crystal semiconductor layer and includes P-type impurities, and a Si film 109, serving as the cap layer, are formed by epitaxial growth on the Nxe2x88x92 collector diffusion layer 112 of the Si epitaxial layer 102. The SiGe-HBT also includes a P+ base polysilicon film 114, which is formed on the region spanning from the lateral surfaces of the SiGe film 108 and the Si film 109 to the upper surface of the Si film 109 and includes a high concentration of P-type impurities, and an N+ emitter polysilicon film 113 including a high concentration of N-type impurities, which is provided over an opening formed in the P+ base polysilicon film 114 including a high concentration of P-type impurities. However, the P+ base polysilicon film 114 and the N+ emitter polysilicon film 113 are electrically separated from each other by an insulating film.
Here, the SiGe film 108 and the Si film 109 are epitaxially grown using MBE, UHV-CVD, or LP-CVD. The region of the Si film 109 directly below the N+ emitter polysilicon film 113 is doped with N-type impurities (phosphorus or arsenic, for example) that have been diffused from the N+ emitter polysilicon film 113 by RTA. That is, the N+ region of the Si film 109 functions as the emitter region of the NPN bipolar transistor, the P+ region of the SiGe film 108 functions as the base region of the NPN bipolar transistor, and the Nxe2x88x92 collector diffusion layer 112, the N+ buried layer 110, and the N+ collector lead layer 111 function as the collector region of the NPN bipolar transistor.
In the process for manufacturing the semiconductor device, after the SiGe film 108 is epitaxially grown on the Si epitaxial layer 102, the Si film 109 is then successively grown epitaxially on the SiGe film 108. The Si film 109 is necessary for preventing Ge contamination of the manufacturing line primarily during the further process steps after the epitaxial growth of SiGe, however, it is possible to form an emitter-base junction (hereinafter, called the xe2x80x9cEB junctionxe2x80x9d) at a desired depth and position in the Si film 109 by adequately selecting the thermal processing conditions for diffusing the N-type impurities in accordance with the film thickness of the Si film 109 and the concentration of the N-type impurities in the emitter polysilicon film 113.
Conventional SiGe-HBTs formed in this way have the advantage over homojunction bipolar transistors made from only a Si layer that impurities do not have to be doped to a high concentration in the emitter region to achieve a large emitter injection efficiency, and they can be expected to have a high current amplification factor (hFE).
FIG. 9 is an energy band diagram for comparing the band structures of Si/SiGe heterojunction bipolar transistor (SiGe-HBT) with graded composition and a Si homojunction bipolar transistor (Si-BT). In the SiGe-HBT, the height of the barrier with respect to holes injected into the emitter region from the base region can be made larger than the height of the barrier with respect to electrons injected from the emitter region into the base region. For this reason, the emitter injection efficiency does not drop even if the impurity concentration of the emitter region is lowered and the impurity concentration of the base region is raised.
Put differently, with SiGe-HBTs the narrow band gap properties of SiGe can be used to achieve a higher current amplification factor than in Si-BTs, even if the emitter region is not doped to a high concentration.
Si has a band gap of approximately 1.1 eV, and Ge has a band gap of approximately 0.7 eV. When a SiGe film includes 10 to 15% Ge, the band gap is between that of Si and Ge, at about 1.0 eV. Thus, monotonically increasing the Ge content in the SiGe film 108 from the emitter side to the collector side (graded composition) creates a graded structure in which the energy band gap Eg becomes continually smaller from the emitter side toward to the collector side, as shown by the solid line in FIG. 9. For this reason, an internal electric field E, as expressed by the following equation (1):
E(eV)=(1.1xe2x88x921.0)/qWxe2x80x83xe2x80x83(1) 
(q: charge amount, W: base width) occurs in the base layer, and the minority carriers that are injected from the emitter into the base can be accelerated by the electric field E. Therefore, higher operation speeds than conventional Si-BTs, in which the minority carriers transit through the base region only by diffusion, can be easily achieved.
However, the above-described conventional SiGe-HBTs have the following problems.
FIG. 10 is a diagram showing the impurity concentration distribution and the change in the Ge content in the depth direction in the cross section taken along the line Xxe2x80x94X shown in FIG. 8. As shown in FIG. 10, the SiGe film 108 is divided into an undoped SiGe buffer layer 108x, and a SiGe graded composition layer 108y into which P-type impurities have been introduced at a high concentration, and which has a continuously changing band gap. A P-type impurity diffusion region 132, serving as the base region, is formed in the upper part the SiGe film 108, and an N-type impurity diffusion region 131, which serves as the emitter region, is formed spanning from the Si film 109 into a portion of the SiGe film 108. That is, the P-type impurity diffusion region 132 and the N-type impurity diffusion region 131 overlap. The reason for this is that during thermal processing for forming the emitter region, N-type impurities from the emitter polysilicon film 113 reach into not only the Si film 109 but also a portion of the SiGe film 108 below the Si film 109.
FIG. 11 is a diagram for illustrating the variations in the diffusion depth of N-type impurities in the SiGe graded composition layer 108y. As illustrated in the drawing, thermal processing conditions can result in large variations in the diffusion depth of the N-type impurities, which determines an EB junction 133. Thus, it is extremely difficult to control the positioning of the EB junction 133 with precision and eliminate variations in that positioning.
As mentioned above, in conventional SiGe-HBTs, the diffusion depth of the emitter region fluctuates, so the Ge content in the EB junction 133 fluctuates in practice. As a result, when for example the Ge content in the EB junction 133 increases, the band gap in the EB junction 133 becomes smaller and thus the collector current is increased, as a result leading to the effect that the current amplification factor is increased. That is, minute fluctuations in the Ge content in the EB junction 133 cause large fluctuations in the current amplification factor, thus making it difficult to obtain a transistor in which the current amplification factor is constant and has few variations.
When the base layer is made thin to increase the speed of the transistor, the grading of the change of the Ge content becomes particularly large, and thus small fluctuations in the position of the EB junction 133 have a noticeable influence on variations in the current amplification factor.
On the other hand, if the EB junction 133 is formed not in the SiGe film 108 but in the Si film 109, then the EB junction 133 is separated from the heterojunction and the effect of the narrow band gap is lost. Thus, a high current amplification factor and the effects of the heterojunction itself can no longer be obtained.
Furthermore, in conventional SiGe-HBTs, there was the problem that it was difficult to control the concentration of boron (B) in the base region.
For example, it is known that when B2H6 is used as the boron source gas, the concentration of boron in the film is reduced (ratio of incorporated B is reduced) together with a reduction in the Ge content, even if the flow rate of B2H6 is constant while depositing the SiGe film. That is, when SiGe-HBTs have a graded SiGe composition, the boron concentration is graded with the same orientation as the grading of the Ge.
FIG. 12 is a diagram showing the impurity concentration distribution and change in the Ge content in the depth direction in a conventional SiGe-HBT when the flow rate of B2H6 is constant. As shown in FIG. 12, it can be understood that along with the reduction in the Ge content in the SiGe graded composition layer 108y, the boron concentration of the P-type impurity diffusion region 132 is also reduced. Then, when variations occur in the position of the EB junction 133, the variations in the concentration of B in the EB junction 133 increase, resulting in increased variations in EB breakdown voltage properties.
In SiGe-HBTs, however, the goal is to lower the base resistance by making the boron concentration in the base region greater than that of ordinary Si-BJTs (or in some cases, greater than the N-type impurity concentration of the emitter region). On the other hand, the EB breakdown voltage drops when the boron concentration of the base region is raised too high, so the boron concentration must be adjusted to keep the base resistance and the EB breakdown voltage within a desired range. In SiGe-HBTs, however, if variations in the EB breakdown voltage increase, then the design margin is narrowed, and there is the risk that it will become difficult to adjust the boron concentration such that the base resistance and the EB breakdown voltage are kept within the desired ranges.
On the other hand, the Ge content in the SiGe film is regulated by the flow rate of the Ge source gas (GeH4, for example), so to compensate for the drop in the amount of incorporated boron that occurs along with the drop in the Ge content (drop in the GeH4 flow rate) the flow rate of B2H6 can in principle be increased to keep a constant boron concentration in the SiGe film. In actuality, however, the GeH4 flow rate and the Ge content in the SiGe film are not proportional, nor are the flow rate of the B2H6 and the boron concentration in the SiGe film proportional, so regulating the gas flow rate in this way would lead to a more complicated manufacturing process.
A primary object of the present invention is to devise a means for inhibiting variations in the band gap of the EB junction, even if there are fluctuations in the position of the EB junction during manufacturing, in a heterojunction bipolar transistor having a structure with a graded band gap for accelerating the carriers, thereby maintaining high operating speeds and obtaining a stable high current amplification factor. Furthermore, the present invention aims to inhibit variations in EB breakdown voltage, while avoiding to make the manufacturing process more complicated.
A first semiconductor device according to the present invention includes a substrate having a first semiconductor layer; a second semiconductor layer provided on the first semiconductor layer, wherein the second semiconductor layer has a smaller band gap than the first semiconductor layer and is made of a mixed crystal semiconductor; and a third semiconductor layer, which is provided on the second semiconductor layer and has a larger band gap than the second semiconductor layer; wherein the semiconductor device functions as a heterojunction bipolar transistor in which at least a portion of the first semiconductor layer is a collector region including first conductive-type impurities; at least a portion of the second semiconductor layer is a base region including second conductive-type impurities; and at least a portion of the third semiconductor layer is an emitter region including the first conductive-type impurities; wherein the second semiconductor layer comprises a graded composition layer having a composition in which the band gap becomes larger in a direction from the collector region toward the emitter region, and a upper layer having a composition in which the band gap change ratio is smaller than the band gap change ratio of the graded composition layer; and an emitter-base junction is formed in the upper layer of the second semiconductor layer.
Thus, because the emitter-base junction is formed within the upper layer of the second semiconductor layer, which is made of a mixed crystal semiconductor, a high current amplification factor resulting from the narrow band gap can be achieved. Also, because the upper layer of the second semiconductor layer has a smaller band gap change ratio than the graded composition layer, even if there are fluctuations in the range to which the first conductive-type impurities are introduced for forming the emitter region, variations in the band gap of the emitter-base junction are reduced, and it is possible to inhibit the width of fluctuation in bipolar transistor properties, such as fluctuations of the current amplification factor.
It is preferable that the composition of the mixed crystal semiconductor in the upper layer of the second semiconductor layer is substantially constant, and the band gap in the upper layer is substantially constant. Thus, even if there are fluctuations in the range to which the first conductive-type impurities are introduced for forming the emitter region, it is possible to better suppress fluctuations of the bipolar transistor properties.
It is preferable that the composition of the mixed crystal semiconductor in the upper layer of the second semiconductor layer is substantially continuously changing, and the band gap of the upper layer changes to become larger in the direction from the collector region toward the emitter region. Thus, it is possible to more effectively achieve carrier acceleration functions, which result from the internal electric field, across the entire base layer.
It is preferable that the second semiconductor layer has a band gap in the upper layer which increases in the direction from the collector region toward the emitter region, and further comprises a top layer, in which the band gap change ratio is larger than the band gap change ratio of the upper layer. Thus, lattice strain resulting from different lattice constants at the border region between the second semiconductor layer and the third semiconductor layer is reduced, so it is possible to suppress crystal defects caused by lattice strain in the second and third semiconductor layers.
It is preferable that the second semiconductor layer is a SiGe layer, the third semiconductor layer is a Si layer, and the Ge content in the upper layer of the second semiconductor layer is in a range of 2 to 8%. Thus, it is easy to achieve both a high current amplification factor resulting from the narrow band gap of the second semiconductor layer, which is made of a mixed crystal semiconductor, and increased base transit speeds due to the graded composition.
It is preferable that the second semiconductor layer is a SiGe layer, the third semiconductor layer is a Si layer, and the Ge content in the top layer of the second semiconductor layer changes not more than 4%. Thus, crystal defects can be prevented, and a high current amplification factor resulting from the narrow band gap of the second semiconductor layer, which is made of a mixed crystal semiconductor, can be achieved easily.
It is also possible that the second semiconductor layer is a mixed crystal semiconductor layer including the three elements of silicon, germanium, and carbon, for example, and that the third semiconductor layer is a Si layer.
It is preferable that the emitter-base junction is positioned substantially in the center of the upper layer of the second semiconductor layer.
It is preferable that the impurity concentration in the graded composition layer of the second semiconductor layer decreases as the band gap increases in the direction from the collector region toward to the emitter region, and the impurity concentration in the upper layer of the second semiconductor layer is substantially constant. Thus, variations in EB breakdown voltage can be inhibited.
It is preferable that the second semiconductor layer is a SiGe layer, and the impurities in the second semiconductor layer are boron (B). Thus, the effects of the present invention can be attained effectively.